The present invention relates to a projection optical system and an image display device using the projection optical system. The image display device is a projection-type device that uses a reflection-type light valve, and is available in various projector formats.
With reference to FIG. 1, a general constitution and function of the projection-type image display device using the reflection-type light valve will be explained.
FIG. 1 is an explanatory diagram that illustrates an optical arrangement of a general projector as the projection-type image display device.
In FIG. 1, reference codes LV, LS, IR, LN, M, CM, and POS denote a light valve, a lamp light source, an integrator rod, a lens for illumination, a mirror, a curved-surface mirror, and a projection optical system, respectively.
The light valve LV is an image display element, and is a reflection type.
The lamp light source LS has a lamp LP, and a reflector RF, and emits illumination light for illuminating the light valve LV.
The integrator rod IR, the lens for illumination LN, the mirror M, and the curved-surface mirror CM constitute an illumination optical system that leads illumination light emitted from the lamp light source LS to the light valve LV.
The integrator rod IR is a light pipe where four mirrors are combined in a tunnel manner, and reflects light that is incident from an entrance part by a mirror surface, and leads it to an exit part.
The projection optical system POS projects reflection light from the light valve LV to a projection surface such as a screen or the like, and enlarges and forms an image displayed on the light valve LV on the projection surface.
The light valve LV is a DMD (Digital Micromirror Device), for example, on which micro mirrors are arranged in an array manner. A normal line of each micromirror is independently changeable in the range of ±12 degrees, for example.
The reflector R reflects light emitted from the lamp LP, and the light converges towards the entrance part of the integrator rod IR. The light incident from the entrance part is repeatedly reflected in the integrator rod IR and led to the exit part, and is emitted as light of uniform illuminance where nonuniformity of illuminance is uniformed from the exit part.
The light emitted from the integrator rod IR illuminates the light valve LV via the lens for illumination LN, the mirror M, and the curved-surface mirror CM of the illumination optical system.
The emitted light from the exit part of the integrator rod IR is taken as a surface light source of uniform illuminance where nonuniformity of a quantity of light is uniformed for the lens for illumination LN and the curved-surface mirror CM of the illumination optical system, and an image of the surface light source is formed on the light valve LV. That is, the light valve LV is illuminated by illumination light of uniform illuminance.
For example, it is defined such that when an inclination angle of a micro mirror on the light valve LV is −12 degrees, for example, light reflected by the micro mirror is incident to the projection optical system POS, and when an inclination angle of a micro mirror on the light valve LV is +12 degrees, light reflected by the micro mirror is not incident to the projection optical system POS, and a positional relationship between the light valve LV and the projection optical system POS is defined. And additionally, an incident direction onto the light valve LV of illumination light from the curved-surface mirror CM is set.
In accordance with pixels of an image to be projected and formed on the projection surface, the inclination of each micro mirror is adjusted, and thereby the image is displayed on the light valve LV.
When an image is displayed on the light valve LV, and the light valve LV is illuminated by the illumination light in this manner, reflection light per micro mirror incident to the projection optical system POS becomes an imaging luminous flux by the projection optical system POS, and is projected and formed on the projection surface as an enlarged image of the image.
Since the light valve LV is illuminated by light of a uniform illuminance distribution, the image on the projection surface as the enlarged image also has the uniform illuminance distribution. Thus, an image is displayed on the projection surface.
The above is an explanation of image display by a general projector.
Recently, a projector, a set position of which to the projection surface is much closer than that of a conventional one, has been known (hereinafter, referred to as ultra-close-range projector).
An ultra-close-range projector has an effect of preventing glare of projection light from entering the eyes of a presenter (speaker, or the like) standing closer to a screen surface (projection surface).
Additionally, the ultra-close-range projector is set away from the audience listening to an explanation made by the presenter, therefore it is possible to remove any negative influence such as exhaust air, noise, or the like from the audience.
Projection optical systems for the ultra-close-range projector are broadly divided into two kinds: a first one that is a conventional coaxial rotationally symmetric projection optical system with a wider angle of view which shortens a distance to a projection surface, and a second one that uses a curved-surface mirror in an optical system.
The first one achieves ultra-close-range projection by using a conventional technique; however, a diameter of a lens close to the screen surface is likely to become large, and the size of a projector is likely to be increased.
On the other hand, the second one maintains the size of a projector small, and has a possibility of achieving ultra-close-range projection.
For example, Japanese Patent number 3727543, Japanese Patent Application publication number (translation for PCT application) 2008-522229, Japanese Patent number 4467609, Japanese Patent Application publication number 2009-145672, Japanese Patent number 4210314, Japanese Patent Application publication number 2007-183671, and Japanese Patent Application publication number 2008-96762 are known as examples using a curved-surface mirror in a projection optical system.
Japanese Patent Number 3727543, Japanese Patent Application publication number 2007-183671, and Japanese Patent Application publication number 2008-96762 disclose examples in which a lens system and a convex mirror are combined, and the main purpose of which is to reduce the thickness of a rear-projection TV.
Japanese Patent Application publication number 2008-522229, Japanese Patent number 4467609, Japanese Patent Application publication number 2009-145672, and Japanese Patent number 4210314 disclose examples in which a lens system and a concave mirror are combined, which achieves ultra-close-range projection.
In a projection optical system using a convex mirror, a diameter of an imaging luminous flux from the convex mirror to a screen monotonously enlarges, as is clear from FIG. 9 of Japanese Patent Application publication number 2007-183671, and so on, for example.
Therefore, considering a case of setting a dust-proof glass for convex mirror protection between the convex mirror and the screen, as to the ultra-close-range projection, enlargement of the diameter of the imaging luminous flux that monotonously enlarges is rapid, and therefore the size of the dust-proof glass is likely to be enlarged, leading to a problem in weight and cost of a projector.
The dust-proof glass is not always necessary; however, if the dust-proof glass is not used, the convex mirror is exposed. Accordingly, dust adheres to a mirror surface, the surface is scratched, or a user who touches the convex mirror and leaves fingerprints on the surface.
The above dust, scratch, and fingerprints have a high possibility of causing deterioration of brightness or image quality of a projected image.
Therefore, the projection optical system using the convex mirror is stored in a housing, and mostly used in the form of a rear projector (See FIG. 25 of Japanese Patent number 3727543, and so on).
On the other hand, in a case of using a concave mirror in the projection optical system, it is possible to converge an imaging luminous flux between the concave mirror and the screen one time. Therefore, if the dust-proof glass is placed between the concave mirror and the screen, it is possible to avoid the dust-proof glass being enlarged.
However, in the ultra-close-range projector, a distance between the concave mirror and the screen is short, and in order to display a large image, an angle of a light beam incident on the screen becomes considerably sharp.
Accordingly, if the concave mirror is used for a front projector (a projector that projects an image from the front with respect to a screen) having the dust-proof glass, the angle of the light beam incident on the dust-proof glass from the concave mirror becomes sharp, as light goes toward an edge part of an image on the screen.
Therefore, transmittance of a dust-proof glass becomes low, as light goes toward an edge part of an image on the screen, and a quantity of light in the vicinity of the edge part of the image also becomes low. That is, the closer to the vicinity of the edge part of the image on a projection screen, the lower the brightness of a projected image becomes.
As to downsizing of a projector, as illustrated in FIG. 13 of Japanese patent number 4210314, it is possible to downsize a projection optical system by inserting a flat fold mirror between a lens system and a concave mirror, and folding a light path. However, an angle of a light beam incident on the fold mirror becomes sharp (not as sharp as the light beam reflected by the concave mirror), and therefore, in the projector where the fold mirror is placed, the quantity of light in the edge part of the image on the screen becomes low also by the fold mirror.
As disclosed in Japanese Patent application publication number 2008-522229, Japanese Patent number 4467609, and Japanese Patent Application Publication number 2009-145672, if the dust-proof glass is placed obliquely, and has an oblique angle to a normal line of the screen, it is possible to slightly moderate the sharpness of an angle of a light beam toward the edge part of the image. However, in particular, in a case of the projection optical system having the fold mirror, if the dust-proof glass is placed obliquely, the dust-proof glass is likely to interfere with the fold mirror, and block a light path toward the concave mirror from the fold mirror. Additionally, in order to avoid the above, the dust-proof glass is likely to be extremely enlarged.
Japanese Patent number 3727543 discloses that “vignetting (light falloff)” by a lens frame of the projection optical system, or the like is reduced, and the size of lenses is minimized by corresponding a position of a lens surface closest to a reflection-type light valve (DMD) to a position of an entrance pupil, and giving the lens surface closest to the DMD an aperture function.
However, in a case of a projector using a reflection-type light valve, unless the position of the entrance pupil is considered, light is blocked by a lens or a mirror of the illumination optical system that illuminates the light valve, and the quantity of light around the image is decreased.